The present invention relates in general to the production of a web of sheet material commonly referred to as a "process" and, more particularly, to a method and apparatus for monitoring and/or calibrating a detector used in measuring one or more characteristics of the process as it is being produced.
In the art of making sheet material, such as paper, coated paper, plastics and the like, it is important to monitor various characteristics of the sheet material or process which is typically manufactured as a relatively fast moving web. Monitoring systems in turn must be monitored and frequently calibrated to provide accurate information for controlling manufacture of the process.
One currently popular form of monitoring webs of sheet material traveling in a direction referred to as the machine direction, is to physically move a monitoring gauge across the web in a direction referred to as the cross direction which is substantially perpendicular to the machine direction. For calibration purposes, movement of a scanning gauge is continued to move the gauge off the process being monitored to a position where it can be exposed to a set of standard conditions, such as a process sample having known characteristics. It is apparent that such scanning measurement ignores the majority of the web since only the material along a zig-zag line is monitored by the traveling gauge. The amount of material which is monitored is further reduced by performing off-process calibration.
In an attempt to more fully monitor a process being manufactured and to speed up web measurement and control, a stationary optical sensor extending continuously across a paper web is disclosed in U.S. Pat. No. 5,071,514. As disclosed in the '514 patent, the stationary optical sensor is calibrated by a closely associated scanning optical sensor which senses discrete regions of the web as it is scanned across the web.
In this arrangement, the stationary sensor can be calibrated on-line using the scanning sensor. The scanning sensor can itself be calibrated off-line, i.e. off the process being measured, without impacting current operation of the stationary sensor. Unfortunately, provision of both a stationary sensor and a scanning sensor is very expensive and complicates the structure and operation of the overall measurement system.
Accordingly, there is a need for an effective and inexpensive arrangement for monitoring and/or calibrating on-line process measuring systems without interrupting the collection of process data. Such a monitoring and/or calibrating arrangement is ultimately required for a commercially viable stationary, non-scanning sensor for a process monitoring system.